Snmr pulse sequence phase cycling

ABSTRACT

Technologies applicable to SNMR pulse sequence phase cycling are disclosed, including SNMR acquisition apparatus and methods, SNMR processing apparatus and methods, and combinations thereof. SNMR acquisition may include transmitting two or more SNMR pulse sequences and applying a phase shift to a pulse in at least one of the pulse sequences, according to any of a variety of phase cycling techniques. SNMR processing may include combining SNMR from a plurality of pulse sequences comprising pulses of different phases, so that desired signals are preserved and undesired signals are canceled.

STATEMENT OF GOVERNMENT SUPPORT

This invention was made with Government support under Agreement No.DE-FG02-08ER84979 awarded by the Department of Energy. The Governmenthas certain rights in this invention.

BACKGROUND

Nuclear Magnetic Resonance (NMR) systems have been in use for many yearsand can be used to provide imaging and/or analysis of a sample beingtested. For example, U.S. Pat. No. 6,160,398, U.S. Pat. No. 7,466,128,U.S. patent application Ser. No. 12/672,503, and U.S. patent applicationSer. No. 12/914,138 describe a variety of NMR technologies, and areincorporated herein by reference. Various different types of NMR includemedical NMR, often referred to as Magnetic Resonance Imaging (MRI), andSurface NMR (SNMR), which provides geophysical techniques for detectingsubsurface liquids in the earth's crust. While there is some overlap inthe technologies that may be applied in MRI and SNMR, the samples beingmeasured and the environments in which measurements are performed aredifferent, leading to many differences in the technologies applied.

In practice, the signals recorded by SNMR instruments can contain acombination of both “desired” and “undesired” signals. The desiredsignals are those particular coherent signals emitted by subsurfaceliquids that can be analyzed to determine the properties of thesubsurface. The undesired signals are any coherent signals thatcomplicate this analysis and may include signals from non-NMR sources aswell as interfering signals from NMR sources. Existing SNMR detectiontechniques are generally useful for detecting desired signals in abackground of white Gaussian noise. Existing SNMR detection techniquesare not as useful for detecting desired NMR signals in the presence ofundesired signals and other undesired interference processes.

SUMMARY

Technologies applicable to SNMR pulse sequence phase cycling aredisclosed, including SNMR acquisition apparatus and methods, SNMRprocessing apparatus and methods, and combinations thereof. Example SNMRacquisition methods include arranging one or more induction coils on thesurface of the Earth, transmitting two or more electrical current pulsesequences on the induction coils, each pulse sequence comprising one ormore oscillating electrical current pulses, and applying a phase shiftto a pulse in at least one of the pulse sequences relative to a pulse inanother of the pulse sequences. The phase shift may be applied accordingto a variety of SNMR pulse sequence phase cycling techniques disclosedherein.

SNMR acquisition methods may be combined with SNMR processing methods insome embodiments. For example, the disclosed SNMR methods may extend todetecting signals on the induction coils after and/or during each of theelectrical current pulse sequences, and linearly combining detectedsignal data corresponding to separate electrical current pulse sequencesto produce combined signal data in which one or more detected signalcomponents are preserved and one or more different detected signalcomponents are reduced or cancelled. The preserved signal components maycomprise NMR signal data, and the reduced or cancelled signal componentscomprise undesired NMR signal data and/or non-NMR signal data.Alternatively, the preserved signal components may comprise undesiredNMR signal data and/or non-NMR signal data, and the reduced or cancelledsignal components comprise NMR signal data.

Example SNMR acquisition systems may comprise systems configured toproduce NMR in underground liquids, including a SNMR phase cyclingcomputer comprising a processor and memory, the SNMR phase cyclingcomputer comprising one or more SNMR phase cycling acquisition modules.The SNMR acquisition modules may be configured to control transmittingof two or more electrical current pulse sequences on induction coilsarrangeable on or above the surface of the Earth, each transmitted pulsesequence comprising one or more oscillating electrical current pulses.The SNMR acquisition modules may be configured to apply a phase shift toa pulse in at least one of the transmitted pulse sequences relative to apulse in another of the transmitted pulse sequences. The phase shift maybe applied according to a variety of SNMR pulse sequence phase cyclingtechniques disclosed herein. Also, SNMR acquisition systems may comprisea variety of additional components such as oscillating waveformgenerator devices, power amplifier(s), one or more transmit switches,one or more induction coils, one or more receive switches, one or morepreamplifiers, and an Analog to Digital (A/D) converter device.

Example SNMR processing methods include coherently combining detectedNMR signal data, such as NMR signal data detected from an undergroundliquid, and including NMR signal data from two or more separateelectrical current pulse sequences, each of the pulse sequencescomprising one or more oscillating electrical current pulses, andwherein the phase of a pulse in at least one of the pulse sequences isshifted relative to a pulse in another of the pulse sequences. SNMRprocessing methods may further include recording a combined NMR signalin which a desired Free Induction Decay (FID) signal is preserved, andundesired signals that are coherent with the timing of the electricalcurrent pulse sequences but independent of the phases of the oscillatingelectrical current pulses are cancelled.

Example SNMR processing systems may comprise a computer equipped with aprocessor and memory, and one or more SNMR phase cycled signal dataprocessing modules configured to coherently combine detected NMR signaldata. The detected NMR signal data may include, for example, datadetected from an underground liquid, and including NMR signal data fromtwo or more separate electrical current pulse sequences, each of thepulse sequences comprising one or more oscillating electrical currentpulses, and wherein the phase of a pulse in at least one of the pulsesequences is shifted relative to a pulse in another of the pulsesequences. The SNMR processing modules may be further configured torecord combined NMR signal data, in which a desired FID signal ispreserved, and undesired signals that are coherent with the timing ofthe electrical current pulse sequences but independent of the phases ofthe oscillating electrical current pulses are cancelled.

Further aspects and variations are discussed in detail below.

BRIEF DESCRIPTION OF THE DRAWINGS

Various features and attendant advantages of the disclosed technologieswill become fully appreciated when considered in conjunction with theaccompanying drawings, in which like reference characters designate thesame or similar parts throughout the several views, and wherein:

FIG. 1 illustrates aspects of an example SNMR system configured toperform SNMR pulse sequence phase cycling;

FIG. 2 is a block diagram illustrating an example computer configured toperform SNMR pulse sequence phase cycling;

FIG. 3 is a flow diagram illustrating example SNMR pulse sequence phasecycling methods;

FIG. 4 illustrates example transmitted pulses, in the top graphs, thatproduce signals illustrated in the bottom graphs, which signals mayreceived, recorded and combined as shown; and

FIG. 5 illustrates example transmitted pulses comprising excitationpulses and refocusing pulses, in the top graphs, that produce signalsillustrated in the bottom graphs, which may be recorded and combined asshown.

DETAILED DESCRIPTION

Prior to explaining embodiments of the invention in detail, it is to beunderstood that the invention is not limited to the details ofconstruction or arrangements of the components and method steps setforth in the following description or illustrated in the drawings. Theinvention is capable of other embodiments and of being practiced andcarried out in various ways. Also, it is to be understood that thephraseology and terminology employed herein are for the purpose of thedescription and should not be regarded as limiting.

Technologies applicable to SNMR pulse sequence phase cycling mayinclude, inter alia, SNMR acquisition apparatus and methods, SNMRprocessing apparatus and methods, and combinations thereof. SNMRacquisition according to this disclosure may include transmitting two ormore SNMR pulse sequences and applying a phase shift to a pulse in atleast one of the pulse sequences relative to a pulse in another of thepulse sequences. The phase shift may be applied according to a varietyof phase cycling techniques. SNMR processing may include combining NMRsignals resulting from a plurality of pulse sequences comprising pulsesof different phases, in such a manner as to preserve desired signals andcancel undesired signals.

FIG. 1 illustrates aspects of an example SNMR system 100 configured toperform SNMR pulse sequence phase cycling. The example SNMR system 100includes a computer 110, function generators 111, 112, ACvoltage/current generator(s) 130, transmit switch(es) 140, inductioncoil(s) 150, receive switch(es) 160, preamplifier(s) 170, and Analog toDigital (AD) converter(s) 120. The induction coil(s) 150 are illustratedover a ground surface 180. A subsurface fluid 190 is illustrated beneaththe ground surface 180. Earth's magnetic field 195 exists over and underthe ground surface 180 and within the subsurface fluid 190.

In FIG. 1, the computer 110 is coupled to function generators 111, 112by connections 113 and 114, respectively. The computer 110 is alsocoupled to AC voltage/current generator(s) 130 by connection 115, totransmit switch(es) 140 by connection 116, to receive switch(es) 160 byconnection 117, and to AD converter(s) 120 by connection 122.Furthermore, function generators 111, 112 are coupled to ACvoltage/current generator(s) 130 by connections 131 and 132,respectively. AC voltage/current generator(s) 130 are coupled totransmit switch(es) 140 by connections 133 and 134. Transmit switch(es)140 are coupled to both ends of the induction coil(s) 141 and 142. Theends of the induction coil(s) 141 and 142 are coupled to receiveswitch(es) 160 by connections 161 and 162, respectively. Receiveswitch(es) 160 are coupled to preamplifier(s) 170 by connections 171 and172. Preamplifier(s) 170 are coupled to AD converter(s) 120 byconnection 121. AD converter(s) 120 are coupled to AD converter(s) 120by connection 121.

In general, with regard to FIG. 1, the SNMR system 100 may be configuredto produce electrical current pulse sequences on the induction coils150. Each electrical current pulse sequence may comprise one or moreoscillating electrical current pulses. When a pulse sequence comprisesmore than one pulse, the pulses may be separated by a pulse separationtime. Also, pulse sequences may be separated by a pulse sequenceseparation time.

The computer 110 may be configured to produce a pulse by selecting apulse phase, and activating the AC voltage/current generator(s) 130. Thecomputer 110 may be configured to select a pulse phase for example byactivating a function generator 111 or 112 corresponding to a desiredpulse phase, so that the selected function generator 111 or 112 providesan input pulse phase to the AC voltage/current generator(s) 130, whichis then amplified by the AC voltage/current generator(s) 130 to producea corresponding pulse on the induction coil(s) 150. The computer 110 mayalso optionally be configured to close one or more transmit switch(es)140 when activating the AC voltage/current generator(s) 130, and openthe transmit switch(es) 140 after activating the AC voltage/currentgenerator(s) 130.

The computer 110 may be configured to produce a pulse sequence byproducing a first pulse, then if additional pulses are included in thesequence, waiting for a predetermined pulse separation time, and thenproducing a next pulse, and repeating until the pulse sequence iscomplete. The computer 110 may be configured to produce two or morepulse sequences by producing a first pulse sequence, then waiting for apredetermined pulse sequence separation time, then producing a nextpulse sequence, and repeating until a desired number of pulse sequencesare complete.

The SNMR system 100 may also be configured to receive and record NMRsignal data received via the induction coil(s) 150. The SNMR system 100may be configured to receive and record NMR signal data after one ormore pulses within a pulse sequence, and/or after completion of a pulsesequence. In some embodiments, the computer 110 may be configured toclose the receive switch(es) 160 after a pulse. The preamplifier(s) 170amplify desired and undesired signals received via induction coil(s)150. The AD converter(s) 120 convert the received and amplified signalsto digital NMR signal data, e.g. by sampling received signals at adesired sampling rate, and the computer 110 or other device equippedwith storage media may be configured to store the digital NMR signaldata.

In some embodiments, the computer 110 may be configured to processdetected NMR signal data, e.g., to combine NMR signal data received andrecorded after one or more pulses within a pulse sequence, and/orreceived and recorded after completion of pulse sequences, in such a waythat preserves desired NMR signal data and cancels undesired NMR signaldata. It will be appreciated that while the computer 110 may beconfigured to perform SNMR processing, in some embodiments SNMRacquisition and SNMR processing may be performed separately, e.g., byfirst performing SNMR acquisition with a SNMR system 100, thenprocessing acquired SNMR data at a later time and/or with a differentcomputing device.

In some embodiments, computer 110 may be programmed with software thatcontrols the generation of pulse sequences and the acquisition of data.A set of data acquisition devices may comprise devices configuredgenerate the control signals for the pulse sequences, such as functiongenerators 111, 112, and AD converter(s) 120 that receive, convertand/or record SNMR signals. The AC voltage/current generator(s) 130 maybe configured to generate one or more current pulses in the inductioncoil(s) 150 in a transmit mode, to induce a coherent precession of NMRspins in the subsurface fluid 190. Optional transmit switch(es) 140 maybe configured to isolate transmitter noise from the receive circuitryduring a receive mode. Induction coil(s) 150 may be arranged on or abovethe surface of the Earth 180, and may be configured to cause a coherentprecession of spins in the subsurface fluid 190 in the Earth's magneticfield 195 and also to detect the NMR magnetic fields generated by thecoherent precession of spins in the subsurface fluid 195. Optionalreceive switch(es) 160 may be configured to isolate the receivepreamplifier(s) 170 from the potentially large voltage on the inductioncoil(s) 150 during transmit mode. Optional preamplifier(s) 170 may beconfigured to amplify the detected NMR signals prior to digitization bythe AD converter(s) 120. The optional transmit switch(es) 140 andreceive switch(es) 160 may comprise active devices such as relays,and/or passive devices such as diodes. Optional tuning capacitors, notshown in FIG. 1, may be used in the transmit mode to increase thetransmitted current in the induction coil(s) 150, and/or in receive modeto increase the amplitude of the NMR signal voltage across the terminalsof the induction coil(s) 150.

In some embodiments, induction coil(s) 150 may comprise an array ofcoils comprising one or more transmit coils, one or more receive coils,and/or one or more combination transmit and receive coils. For example,induction coil(s) 150 may comprise one transmit coil and multiplereceive coils. Induction coil(s) 150 may comprise one combinationtransmit and receive coil, and multiple receive coils. Induction coil(s)150 may comprise multiple combination transmit and receive coils. Theseand other multicoil arrangements may be configured in some embodimentsas will be appreciated. Multicoil arrangements are useful forlocalization of subsurface fluids 190, as described for example in U.S.Pat. No. 7,466,128, which is incorporated by reference.

Any combination of hardware and software that enables the acquisitionand processing of NMR signals from subsurface liquids in the Earth'smagnetic field is suitable to implement embodiments of this disclosure.An architecture to implement the disclosed methods could comprise, forexample, elements illustrated in FIG. 1, such as an AC voltage andcurrent generator 130, a digital control system implemented at least inpart by computer 110, a transmit switching circuit including transmitswitch(es) 140, a receive switching circuit including receive switch(es)160, a multi-channel receive circuit including, e.g., a plurality ofinduction coils 150, preamplifier(s) 170, a digital acquisition systemincluding AD converter(s) 120, a digital storage device which may beimplemented within computer 110 or other digital storage device, and adigital computer 110 equipped with pulse sequence control softwareand/or SNMR processing software. The switching circuits may transition asystem such as 100 between a transmit-mode, when the coil(s) 150 areconnected to the transmit circuit, and receive-mode when the coil(s) 150are connected to the receive circuit. In a single acquisition sequence,the transmit circuit directs an AC current pulse or pulses withcontrolled amplitude and phase (alternating at the Larmor frequency)through the induction coil(s) 150 in short succession. As quickly aspossible after a given transmit pulse, and before the next pulse, theswitching circuits may transfer the induction coil(s) 150 into a single-or multi-channel receive circuit. The data acquisition system may thenrecord the voltages on the receive circuit (including the surfacecoil(s) 150), and may record this received NMR signal data following thetransmit pulse on the digital storage device. To form a complete cycledset, an acquisition sequence may be repeated one or more times, changingthe phase of one or more transmit pulses between each acquisitionsequence. After a complete cycled set corresponding to a NMR measurementis acquired, the signals recorded from each acquisition sequence may belinearly combined through digital processing.

In general, a SNMR measurement may be collected by transmitting one ormore pulses of alternating current through a wire loop on the Earth'ssurface. The alternating current may be tuned to the Larmor frequency ofhydrogen nuclei, and may generate a magnetic field in the subsurfacebeneath the coil(s) alternating at the Larmor frequency. The alternatingmagnetic field radiates into the Earth and modifies the nuclearmagnetization state of hydrogen present in fluids at depth. Atequilibrium, the net nuclear magnetization is aligned with Earth'sbackground magnetic field along the so-called longitudinal axis. Thetransmitted alternating magnetic field perturbs the magnetization fromthis equilibrium alignment so that some component of the nuclearmagnetization rotates into the transverse “xy” plane. Once rotated fromequilibrium, the magnetization relaxes over time back to the equilibriumstate over time, decaying from the transverse plane and re-growing alongthe longitudinal axis. The rotation of the magnetization by thetransmitted pulse(s) and subsequent relaxation to equilibrium aredescribed by the phenomenological Bloch equations. The evolution of themagnetization under the Bloch equations depends on several variablesincluding the amplitude of the transmitted field, the duration andtiming of the transmitted field, the phase of the transmitted field, thelongitudinal relaxation time T1, FID relaxation rate T2*, and/or thespin-spin relaxation time T2 of the hydrogen nuclei under investigation.

An NMR signal is generated by the presence of coherent transversemagnetization following a transmit pulse. The transverse magnetizationgenerates a magnetic field, which oscillates at the Larmor frequency,and generally has a phase related to the phase of one or more of thetransmitted pulses. The SNMR instrumentation records the NMR signal bymonitoring the voltage on the surface loop. Identical measurements maybe repeated to improve signal to noise; measurements using variedtransmit currents may be used to modulate the contribution of signalsfrom groundwater at different depths. Spatial inversion techniques maybe used to isolate NMR signal contributions from different depth rangesor different locations in a 2D or 3D model of the subsurface, asdescribed in U.S. Pat. No. 7,466,128.

Measurement schemes with one or more excitation pulses may be used toprobe different types of NMR responses and properties. In a single pulsemeasurement, a single pulse rotates a component of the magnetizationinto the transverse plane. The signal produced as this coherenttransverse magnetization relaxes to equilibrium is called the FreeInduction Decay (FID) signal. In the single pulse sequence, the pulsesequence is repeated only after a delay period that is sufficiently longto allow the longitudinal relaxation process of liquid hydrogen samplesin the subsurface to relax to their steady state. The FID signal can beused to determine the quantity of subsurface water content and theeffective transverse relaxation time T2*. Double pulse sequences may beused to probe other relaxation times, such as T1 and/or T2. The firstpulse rotates a component of the magnetization into the transverseplane; a second pulse transmitted after a controlled delay furthermodifies and rotates the magnetization state so that the recorded signalfollowing the second pulse contains information about the decay times T1and/or T2.

For single-pulse measurements, the desired signal is the FID signal,which is the only NMR signal contained in the measurement. Undesiredcoherent signals in a single-pulse measurement may be associated withnon-NMR processes including instrumentation artifacts as well as theinductive response of the conductive earth following the termination ofthe transmit pulse. Multiple-pulse measurements can also containundesired NMR signals. In addition to non-NMR artifacts, multiple-pulsemeasurement sequences can produce multiple NMR signals, some of whichare undesired due to the fact that they complicate accurate extractionof the decay times of interest T1 and/or T2.

Embodiments of the present disclosure may take advantage of the factthat the phase of a certain coherent signals is dependent upon specificcontrollable parameters, while the phase of other coherent signals isindependent of these parameters. Specifically for SNMR measurements, thephase of certain coherent signals will be linearly correlated with thephase of one or more transmit pulses, while the phase of other coherentsignals may be constant, negatively correlated, or otherwise independentof the transmit pulse. As an example, in FIG. 4, the top left graphillustrates a transmitted pulse P(t) that may produce signalsillustrated in the bottom left graph, including one signal SA(t) thathas the same phase as P(t), and a second signal SB(t) that has aconstant phase independent of P(t). Introducing a 180 degree phase shiftto P(t), as shown at top right, will likewise introduce a 180 degreephase shift to SA(t), as shown bottom middle, but will not change thephase of SB(t). Thus, if between a pair of independent measurements, thephase of P(t) is shifted by 180 degrees, the phase of SA(t) will also beshifted by 180 degrees while the phase of SB(t) will be constant. Bysubtracting this pair of phase-cycled signals, it is then possible topreserve SA(t) while eliminating SB(t), as show bottom right. Thus byphase-cycling one transmit pulse, it is possible to isolate signals thatare phase-correlated with that pulse from those signal which are notphase-correlated with that particular pulse.

In some embodiments, the phase of each transmitted pulse may be definedrelative to the phase of an un-modulated reference sinusoid at theselected transmitting frequency, wherein the phase of the referencesinusoid does not change with respect to the timing of various appliedpulse sequences associated with a measurement. For example, one maydefine a reference sinusoid such that it has a phase of 0 degrees suchthat the reference sinusoid has a zero crossing at time t=0. Theabsolute phase is unimportant, as this can always be removed in postprocessing. The important relationship is the relative phases of thevarious pulses in the applied pulse sequences.

The phase of a transmitted pulse may be controlled and changed in anumber of ways. For example, if the transmitted pulses are generatedusing local oscillators, then the transmitted phase may be controlled byusing multiple oscillators with different phases, for example, twooscillators such as function generator 111 and 112, one with a phase of0 degrees and one with a phase of 180 degrees, and switching between thedifferent oscillators to produce different pulses. In another example,if the transmitted pulses are generated by a computer-controlled digitalor analog output device, then the phase of each transmitted pulse may becontrolled by software.

Instrument switching artifacts are one type of undesired signal that mayinterfere with SNMR measurements. Embodiments of the present disclosuremay also be employed remove undesired switching artifacts that arecoherent, repeatable, and have no phase dependence on the transmitpulse. The example in FIG. 4 describes the application of the presentdisclosure to preserve a desired NMR signal while suppressing anundesired instrumentation artifact. In FIG. 4, the desired signal,denoted SA, is an NMR signal that has a phase linearly correlated withtransmit pulse. In FIG. 4 the undesired signal, denoted SB, is arepeatable undesired instrumentation artifact signal whose phase isuncorrelated with the transmitted pulse. The transverse magnetizationsignal resulting from the rotation of longitudinal magnetization intothe transverse plane by a transmit pulse is correlated in phase with thephase of the transmit pulse; under non-resonance conditions the phase ofthis signal is the same as that of the transmit pulse. Thus cycling thephase of any transmitted pulse in a pair of measurements will also cyclethe phase of the transverse magnetization rotated from the longitudinalaxis by that pulse. On the other hand, coherent and repeatable switchingartifacts that do not change phase with the transmit pulse, willmaintain constant phase between the measurement pairs. Using a pair ofmeasurements in which the transmit pulse is cycled between ø and ø+180°,provides two recorded signals pairs that can be subtracted to isolateswitching artifacts from the desired NMR signal.

FIG. 2 is a block diagram illustrating an example computer 110configured to perform SNMR pulse sequence phase cycling. As discussed inconnection with FIG. 1, the computer 110 may be configured to producepulse sequences, to receive and record resulting NMR signal data, and/orto perform processing of NMR signal data.

Computing device 110 may include for example a processor 210, memory220, system bus 230, one or more drives 240, user input interface 250,output peripheral interface 260, and network interface 270. Drives 240may include, for example, a compact disk drive 241 which accepts anoptical disk 241A, a so-called hard drive 242, which may employ any of adiverse range of computer readable media, and a flash drive 243 whichmay employ for example a Universal Serial Bus (USB) type interface toaccess a flash memory 243A. Drives may further include network drivesand virtual drives (not shown) accessed via the network interface 270.

The drives 240 and their associated computer storage media providestorage of computer readable instructions, data structures, programmodules and other data for the computer system 110. For example, a harddrive 242 may include an operating system 244, application programs 245,program modules 246, and database 247. Software aspects of thetechnologies described herein may be implemented, in some embodiments,as computer readable instructions stored on any of the drives 240 or onnetwork 272, which instructions may be loaded into memory 220, forexample as modules 223, and executed by processor 210.

Computer system 110 may further include a wired or wireless inputinterface 250 through which selection devices 251 and input devices 252may interact with the other elements of the system 110. Selectiondevices 251 and input devices 252 can be connected to the inputinterface 250 which is in turn coupled to the system bus 230, allowingdevices 251 and 252 to interact with processor 210 and the otherelements of the system 110. Interface and bus structures that may beutilized to implement 250 may include for example a Peripheral ComponentInterconnect (PCI) type interface, parallel port, game port and a wiredor wireless Universal Serial Bus (USB) interface.

Selection devices 251 such as a mouse, trackball, touch screen, or touchpad allow a user to select among desired options and/or data views thatmay be output by the computer 110, for example via the display 262.Input devices 252 can include any devices through which commands anddata may be introduced to the computer 110. For example, in someembodiments the AD converter(s) 120 may be coupled to the computer 110as an input device 252, and data received from the AD converter(s) 120may be stored in drives 240. Other example input devices 252 include akeyboard, an electronic digitizer, a microphone, a joystick, game pad,satellite dish, scanner, media player, mobile device, or the like.

Computer system 110 may also include an output peripheral interface 260which allows the processor 210 and other devices coupled to bus 230 tointeract with output devices such as the function generators 111, 112,the AC voltage/current generator(s) 130, the transmit switches 140, thereceive switches 160, and optionally a Digital to Analog (DA) converteras discussed further herein. Other example output devices includeprinter 261, display 262, and speakers 263. Interface and bus structuresthat may be utilized to implement 260 include those structures that canbe used to implement the input interface 250. It should also beunderstood that many devices are capable of supplying input as well asreceiving output, and input interface 250 and output interface 260 maybe dual purpose or support two-way communication between componentsconnected to the bus 230 as necessary.

Computing system 110 may operate in a networked environment usinglogical connections to one or more computers. By way of example, FIG. 2shows a LAN 271 connection to a network 272. A remote computer may alsobe connected to network 271. The remote computer may be a personalcomputer, a server, a router, a network PC, a peer device or othercommon network node, and can include many or all of the elementsdescribed above relative to computing system 110. Networkingenvironments are commonplace in offices, enterprise-wide area networks(WAN), local area networks (LAN), intranets and the Internet.

When used in a LAN or WLAN networking environment, computing system 110is connected to the LAN through a network interface 270 or an adapter.When used in a WAN networking environment, computing system 110typically includes a modem or other means for establishingcommunications over the WAN, such as the Internet or network 272. Itwill be appreciated that other means of establishing a communicationslink between computers may be used.

In some embodiments, computing system 110 may include modules 246 and/or223 comprising, inter alia, one or more SNMR phase cycling acquisitionmodules, and one or more SNMR phase cycled signal data processingmodules, which may be referred to herein as SNMR acquisition modules andSNMR processing modules, respectively.

The SNMR acquisition modules may be configured to control transmittingof two or more electrical current pulse sequences on induction coilsarrangeable on or above the surface of the Earth. For example, the SNMRacquisition modules may be configured to control the phases of pulseswith each pulse sequence, the time between pulses, the number of pulses,the number of pulse sequences, and the time between pulse sequences. TheSNMR acquisition modules may be configured receive a pulse sequenceselection or configuration from a user input, and may control the two ormore electrical current pulse sequences according to the user selection.The SNMR acquisition modules may be configured to send control signalsto the various devices illustrated in FIG. 1 to control pulse sequencetransmission.

In some embodiments, the SNMR acquisition modules may also be configuredto control receiving and recording signal data received in response totransmitted pulse sequences. For example, the SNMR acquisition modulesmay be configured to operate receive switches 160, to place theacquisition system 100 in a receive mode to detect signals on theinduction coils after and/or during each of the electrical current pulsesequences. Detected signals may be converted to signal data by the ADconverter(s) 120, and the signal data may be recorded in a memory of thecomputing device 110 or elsewhere.

In some embodiments, SNMR processing modules may be configured tolinearly combine detected signal data corresponding to separateelectrical current pulse sequences to produce combined signal data inwhich one or more detected signal components are preserved and one ormore different detected signal components are reduced or cancelled. Thepreserved signal components may comprise, for example, NMR signal data,such as desired NMR data, and the reduced or cancelled signal componentsmay comprise undesired NMR signal data and/or non-NMR signal data.Alternatively, the preserved signal components comprise undesired NMRsignal data and/or non-NMR signal data, the reduced or cancelled signalcomponents comprise NMR signal data.

SNMR processing modules may be configured to process NMR data that isacquired according to the SNMR acquisition techniques discussed herein.For example, SNMR processing modules may be configured to identify NMRdata corresponding to a plurality of different phase-shifted pulsesequences that correspond to a single NMR measurement, and to combinethe identified NMR data. Similarly, SNMR processing modules may beconfigured to identify NMR data corresponding one or more specificpulses within a pulse sequence, and to combine such identified NMR datawith NMR data from a corresponding, phase-shifted pulse from anotherpulse sequence. In some embodiments, the SNMR processing modules may beconfigured to preserve desired NMR signal data and cancel undesired NMRsignal data. For example, SNMR processing modules may be configured tocoherently combine detected NMR signals corresponding to separateelectrical current pulse sequences to produce a combined NMR signal inwhich a desired FID signal is preserved, and undesired signals that arecoherent with the timing of the electrical current pulse sequences butindependent of the phases of the oscillating electrical current pulsesare cancelled. Embodiments configured for the opposite operation arealso possible, namely cancelling desired NMR signal data and preservingundesired NMR signal data. In some embodiments, SNMR processing modulesmay also be configured to perform additional processing operations, suchas applying linear spatial inversion processing, non-linear spatialinversion processing, or correlation-based spatial processing, tolocalize detected NMR signals from underground liquids.

In some embodiments, multiple pulse sequences transmitted by an SNMRsystem 100 may be associated with a single SNMR measurement. Forexample, the pulse sequences may be designed to produce NMR data that iscombined into a combined data set. The combined data set represents theend product of the single SNMR measurement, wherein the single SNMRmeasurement is obtained using a plurality of SNMR pulse sequences. NMRsignal data from a plurality of SNMR pulse sequences may be combinedaccording to SNMR processing techniques disclosed herein.

In some embodiments, each of the transmitted pulse sequences maycomprise one or more oscillating electrical current pulses, and each ofthe oscillating electrical current pulses has a phase of oscillationrelative to the other oscillating electrical current pulses in the twoor more electrical current pulse sequences. For example, a phase of asecond, third, or any subsequent pulse in a first pulse sequence mayhave a same phase of oscillation as a first pulse in the pulse sequence,or may be phase shifted by any amount between 0 and 360 degrees,relative to the first pulse. Similarly, a first, second, or anysubsequent pulse in a second, third, or any subsequent pulse sequencemay be phase shifted by any amount between 0 and 360 degrees, relativeto a pulse in the first pulse sequence.

In some embodiments, the SNMR acquisition modules may be configured toapply phase shift(s) to the transmitted oscillating electrical currentpulses in the transmitted pulse sequences. The SNMR acquisition modulesmay be configured to apply the phase shift by switching an ACvoltage/current generator 130 input between different oscillatingwaveforms. In some embodiments, oscillating waveforms may be provided byoscillating waveform generator devices. The oscillating waveformgenerator devices may comprise, for example, function generators such as111 and 112. Another example of oscillating waveform generator devicesincludes a computer, e.g., computer 110, equipped with waveformgenerating software and a DA signal converter. For example, the SNMRacquisition modules may either include or access waveform generatingsoftware that produces digital waveform(s), which are converted toanalog waveform(s) by the DA signal converter. The SNMR acquisitionmodules may apply the phase shift by switching an AC voltage/currentgenerator 130 input between oscillating waveforms produced by thewaveform generating software.

In some embodiments, the phase shift applied by the SNMR acquisitionmodules may be relative to a phase of at least one of the transmittedoscillating electrical current pulses in another of the transmittedpulse sequences. Example pulse sequence configurations are discussedbelow.

In some pulse sequence configurations at least two transmittedelectrical current pulse sequences may be applied, each pulse sequenceconsisting of a single oscillating electrical current pulse. The phaseof a single oscillating electrical current pulse applied in at least oneof the single pulse sequences may differ by substantially 180 degreesfrom the phase of a single oscillating electrical current pulse appliedin at least another of the single pulse sequences. This approach may bereferred to herein as FID phase cycling to suppress signals withconstant envelope and phase functions, or “FID phase cycling”.

In some embodiments, a FID phase cycling pulse sequence consisting of asingle uninterrupted transmit pulse may be applied twice. In the secondapplication of the pulse sequence, the phase of the transmit pulse maybe shifted by substantially 180 degrees (also referred to as a phaseshift of “pi”) with respect to the phase of the transmit pulse in thefirst application of the pulse sequence. Each single pulse sequence maybe followed by detection and recording of the resultant desired FIDsignal.

SNMR processing modules may be configured to process the data sets fromthe two applied FID phase cycling sequences, e.g. by coherentlysubtracting the data sets, yielding a single linearly combined data set.Since the phase of the NMR FID signal tracks the phase of thetransmitted pulse, in the linearly combined data set the desired NMR FIDsignals add constructively and are hence preserved. Any instrumentationrelated artifact signals that are the same for the two applied signals,including electronic switching transients that are independent of thetransmitted pulse phase, will be cancelled to zero in the linearlycombined data set.

In some pulse sequence configurations, at least two transmittedelectrical current pulse sequences may be applied, wherein at least twoof the transmitted electrical current pulse sequences each consist of Noscillating electrical current pulses, wherein N is greater than one,and wherein each of the pulse sequences are separated by a time delay.The phase of any i^(th) oscillating electrical current pulse applied inat least one of the N-pulse pulse sequences may differ by substantially180 degrees from the phase of an i^(th) oscillating electrical currentpulse applied in at least another of the N-pulse pulse sequences. Someexamples of this arrangement are provided below.

Regarding the time delay between pulses, a time delay separating theoscillating electrical current pulses in each of the pulse sequences mayfor example comprise a time delay that is shorter than the time for anunderground liquid to achieve substantially complete longitudinal and/ortransverse relaxation. Relaxation is an asymptotic process and therefore“complete relaxation” may be difficult to assess, and so a relaxationamounting to a substantially complete relaxation may be, e.g., a 75% orgreater relaxation level.

In some pulse sequence configurations, at least two transmittedelectrical current pulse sequences may be applied, each pulse sequenceconsisting of two oscillating electrical current pulses. The phase of afirst oscillating electrical current pulse applied in at least one ofthe two-pulse pulse sequences may differ by substantially 180 degreesfrom the phase of a first oscillating electrical current pulse appliedin at least another of the two-pulse pulse sequences.

A pulse sequence configuration using two oscillating electrical pulses,in which phase cycling can be applied, is referred to herein aspseudo-saturation-recovery phase cycled pulse sequence or“pseudo-saturation-recovery”. A pseudo-saturation-recovery pulsesequence may comprise the transmission of two successive pulses ofapproximately the same pulse moment or transmitted energy, wherein thetwo pulses are separated by a time delay that is less than the timerequired for the targeted NMR processes to achieve complete longitudinalrelaxation. In one embodiment of the pseudo-saturation-recoverysequence, the phase of the second pulse in the two-pulse sequence may beshifted by substantially 180 degrees with respect to the phase of thefirst pulse, in an attempt to limit the influence of non-uniform tipangles. In this SNMR pulse sequence, the desired NMR signals are the FIDsignals resulting from the action of the first and second pulses on thelongitudinal magnetization state immediately prior to the pulse. Theproduced data are useful for estimating the abundance of subsurfaceliquids and also the T2* and/or T1 relaxation processes of the detectedsubsurface fluids.

In some embodiments, pseudo-saturation-recovery phase cycling may beapplied to a two pulse sequence to preserve desired FID signals afterthe first and/or second pulse, and cancel or reduce undesired signalsthat exhibit a constant phase. Such undesired signals may includeinstrumentation or electronics-related transient signals that exhibit asignal or response that is independent of the transmitted pulse. Apseudo-saturation-recovery pulse sequence may consist of twouninterrupted transmit pulses, with a finite delay between the twopulses, wherein the transmitted energies of the first and second pulsesmay be approximately equal or different, and NMR signal acquisition mayoccur after the first and/or second pulses. The phase shift between thefirst and second pulses may be arbitrary. This two-pulse sequence may beapplied twice. In the second application of the two-pulse sequence, thephase of first transmitted pulse may be shifted by substantially 180degrees with respect to the phase of the first transmitted pulse in thefirst application of the sequence, and phase of the second transmittedpulse may be shifted by substantially 180 degrees with respect to thephase of the second transmitted pulse in the first application of thesequence. Each two-pulse sequence may be followed by detection andrecording of the resultant desired FID signal.

SNMR processing modules may be configured to process the data sets fromthe pseudo-saturation-recovery type acquisition. The data sets from thetwo applied sequences may be subtracted, yielding a single linearlycombined data set. Since the phase of the NMR FID signals tracks thephase of the transmitted pulses, in the linearly combined data set thedesired NMR FID signals add constructively and are hence preserved. Anyundesired signals that are the same for the two applied signals,including electronic switching transients that are independent of thetransmitted pulse phase, will be cancelled to zero in the linearlycombined data set.

In some pulse sequence configurations, at least four transmittedelectrical current pulse sequences may be applied, each pulse sequenceconsisting of two oscillating electrical current pulses. Each of thefour two-pulse pulse sequences may comprise a first oscillatingelectrical current pulse with a first phase, and a second oscillatingelectrical current pulse with a second phase. In one of the fourtwo-pulse pulse sequences, the first phase and second phase may besubstantially both at a same defined phase. In another of the fourtwo-pulse pulse sequences, the first phase may be substantially thedefined phase, and second phase may be shifted substantially 180 degreesfrom the defined phase. In another of the four two-pulse pulsesequences, the first phase may be shifted substantially 180 degrees fromthe defined phase, and second phase may be substantially the definedphase. In another of the four two-pulse pulse sequences, the first phasemay be shifted substantially 180 degrees from the defined phase, andsecond phase may be shifted substantially 180 degrees from the definedphase. This approach may be referred to herein as 4-steppseudo-saturation-recovery phase cycling to suppress signals withconstant envelope and phase, and remnant FID signals from the first andsecond pulses, or “4-step pseudo-saturation-recovery”.

In some embodiments, a 4-step phase cycling pulse sequence may applyfour two-pulse sequences to preserve desired FID signals after the firstand/or second pulse, and cancel or reduce undesired signals that exhibita constant phase. Such undesired signals may include instrumentation orelectronics-related transient signals that exhibit a signal or responsethat is independent of the transmitted pulse. In addition, a separatelinear combination of the acquired data may preserve the desired FIDinitiated by the second pulse while simultaneously canceling and remnantand overlapping portion of the FID signal initiated by the first pulse.

In some embodiments, a 4-step phase cycling pulse sequence may consistof two uninterrupted transmit pulses, with a finite delay between thetwo pulses, where in the transmitted energies of the first and secondpulses may be approximately equal or different. NMR signal acquisitionmay occur after the first and/or second pulses. This two-pulse sequencemay be applied four times. In the first application of the two-pulsesequence, the transmitted phases of the first and second pulses may besubstantially identical. In the second application of the two-pulsesequence, the phase of first transmitted pulse may be shifted bysubstantially 180 degrees with respect to the phase of the firsttransmitted pulse in the first application of the sequence. In the thirdapplication of the two-pulse sequence, the phase of second transmittedpulse may be shifted by substantially 180 degrees with respect to thephase of the second transmitted pulse in the first application of thesequence. In the fourth application of the two-pulse sequence, the phaseof first transmitted pulse may be shifted by substantially 180 degreeswith respect to the phase of the first transmitted pulse in the firstapplication of the sequence, and phase of second transmitted pulse maybe shifted by substantially 180 degrees with respect to the phase of thesecond transmitted pulse in the first application of the sequence. Foursets of data may be received and recorded, and the relative phase shiftsof the first FID signal φ_(FID1) and second FID signal φ_(FID2) for eachdata set for each data set, denoted as (φ_(FID1), φ_(FID2)) may be asfollows: A1=(0, 0), A2=(pi, 0), A3=(0, pi), A4=pi, pi). The order inwhich the four sequences are acquired is arbitrary. On sequence A1, thephase of the first transmit pulse relative to the second transmit pulseis also arbitrary.

SNMR processing modules may be configured to process the data sets from4-step pseudo-saturation-recovery by linearly combining the four sets ofdata in different ways to isolate different desired and/or undesiredsignals. For example, in a first linear combination C1, the fourth dataset A4=(pi, pi) may be subtracted from the first data set A1=(0, 0) toyield a single combined data set in which the FID signals initiated bythe first and second transmit pulses are preserved with zero phase shiftC1=(0, 0), and any undesired signals with constant envelope and phaseincluding instrumentation transients may be canceled or reduced.

In a second combination C2, the second data set A2=(pi, 0) may besubtracted from the third data set A3=(0, pi) to yield a single combineddata set in which the FID signals initiated by the first and secondtransmit pulses are preserved with phase shifts C2=(0, pi), and anyundesired signals with constant envelope and phase includinginstrumentation transients are canceled or reduced.

In a third combination C3, the third data set A3=(0, pi) may besubtracted from the first data set A1=(0, 0) to yield a single combineddata set in which the FID signal initiated by the second transmit pulseis preserved with a phase of 0 degrees, and any undesired signals withconstant envelope and phase following the second pulse, includinginstrumentation artifacts and/or any remnant FID signal initiated by thefirst pulse, may be canceled or reduced. This result is denoted C3=(X,0), wherein the “X” indicated that the first FID signal has beencancelled, and the phase of the combined second FID signal is zero.

In a fourth combination C4, the second data set A2=(pi, 0) may besubtracted from the fourth data set A4=(pi, pi) to yield a singlecombined data set in which the FID signal initiated by the secondtransmit pulse is preserved with a phase shift of pi, and any undesiredsignals with constant envelope and phase following the second pulse,including instrumentation artifacts and/or any remnant FID signalinitiated by the first pulse, may be canceled or reduced. This result isdenoted C4=(X, pi), wherein the “X” indicated that the first FID signalhas been cancelled, and the phase of the combined second FID signal ispi.

In a fifth combination C5, the second data set A2=(pi, 0) may besubtracted from the first data set A1=(0, 0) to yield a single combineddata set in which the FID signal initiated by the first transmit pulseis preserved with a phase shift of 0, and any undesired signals withconstant envelope and phase following the second pulse, includinginstrumentation artifacts and/or any remnant FID signal initiated by aprevious application of the second pulse, may be canceled or reduced.This result is denoted C5=(0, X), wherein the “X” indicated that thesecond FID signal has been cancelled, and the phase of the combinedfirst FID signal is 0.

In a sixth combination C6, the fourth data set A4=(pi, pi) may besubtracted from the third data set A3=(0, pi) to yield a single combineddata set in which the FID signal initiated by the first transmit pulseis preserved with a phase shift of pi, and any undesired signals withconstant envelope and phase following the second pulse, includinginstrumentation artifacts and/or any remnant FID signal initiated by aprevious application of the second pulse, may be canceled or reduced.This result is denoted C6=(pi, X), wherein the “X” indicated that thesecond FID signal has been cancelled, and the phase of the combinedfirst FID signal is pi.

Additional linear combinations of the acquired data may be formulated soas to preserve and/or suppress other specific desired NMR signals and/orinstrumentation artifacts. For example, in a seventh combination C7 thefirst data set A1=(0, 0) may be added to the fourth data set A4=(pi, pi)to yield a single combined data set in which the FID signal initiated bythe first transmit and second transmit pulses are canceled, and anynon-NMR signals with constant envelope including instrumentationartifacts, are preserved. Similarly, in an eighth combination C8 thesecond data set A2=(pi, 0) may be added to the third data set A3=(0, pi)to yield a single combined data set in which the FID signal initiated bythe first transmit and second transmit pulses are canceled, and anynon-NMR signals with constant envelope including instrumentationartifacts, are preserved. The combinations C7 and C8 are useful forcharacterizing the non-NMR transient responses that may be related toinstrumentation or other geophysical phenomena.

The first six linear combinations described above yield six combinationsof the desired FID signals while undesired instrumentation artifacts andother undesired signals with constant envelope and phase are canceled orreduced: C1=(0, 0), C2=(0, pi), C3=(X, 0), C4=(X, pi), C5=(0, X),C6=(pi, X). These six linear combinations {C1, C2, C3, C4, C5, C6} mayalso be linearly combined in different ways to further enhance one orboth of the desired FID signals. For example, the combinations C1 and C2may be further combined via the equation D1=(C1+C2)/2, such that thefirst FID signal is preserved with zero phase shift and white Gaussianmeasurement noise is reduced by a factor of sqrt(2). Similarly thecombinations C3 and C4 may be further combined via the equationD2=(C3−C4)/2, such that the second FID signal is preserved with zerophase shift, any remnant FID signal initiated by the first pulse iscancelled, and white Gaussian measurement noise is reduced by a factorof sqrt(2).

Additional combinations of the acquired data (A1, A2, A3, A4) and/orlinear combinations of the acquired data (C1, C2, etc. . . . ) may bedeveloped and applied to enhance other desired NMR signal processes ornon-NMR signals of interest, and suppress specific undesired NMR andnon-NMR signals.

Another pulse sequence configuration using two oscillating electricalpulses, in which phase cycling can be applied, is referred to herein asspin-echo phase cycled pulse sequence or “spin-echo”. The spin-echopulse sequences may comprise an oscillating electrical currentexcitation pulse, followed by a time delay, followed by an oscillatingelectrical current refocusing pulse. NMR signal acquisition may occurafter the first and/or second pulses. The phase of an oscillatingelectrical current excitation pulse applied in at least one of thespin-echo pulse sequences may differ by substantially 180 degrees fromthe phase of an oscillating electrical current excitation pulse appliedin at least another of the spin-echo pulse sequences. This approach maybe referred to herein as spin echo phase cycling to isolate spin echoesfrom FID signals or “spin echo”.

In some embodiments, a spin echo pulse sequence may comprise two pulsesthat may have different duration or amplitude, and may be transmitted insuccession separated by a delay time. The initial pulse acts to rotate acomponent of the magnetization from the longitudinal plane into thetransverse plane. The magnetization signal in the transverse planeproduced by this pulse decays over time as the magnetization loses phasecoherence in an inhomogeneous background field. The second pulse, calledthe refocusing pulse, applied after a delay time alters themagnetization state and causes a portion of the transverse magnetizationto refocus, forming a so-called spin echo signal, centered in time atapproximately twice the delay time. The amplitude of the echo signal isa function of the delay time and the relaxation time T2. By repeatingthe measurement using different delay times it is possible to determineT2.

The refocusing pulse may not only create the spin echo signal, but alsoother undesired NMR signals. This is especially true in SNMRmeasurements where the transmitted B1 field can vary widely over thedetectable depth range in a near surface aquifer. In addition torefocusing a portion of the transverse magnetization from the firstpulse to form an echo signal, the second pulse can also rotate newmagnetization from the longitudinal axis into the transverse plane,creating a secondary transverse magnetization FID signal. The secondarytransverse magnetization FID signal may overlap and interfere(constructively or deconstructively) with the desired spin echo signal.

The phase of the secondary transverse magnetization signal, on the otherhand, will be correlated with the phase of the refocusing pulse andindependent of the phase of the initial pulse. Thus maintaining aconstant phase ø for the refocusing pulse, and cycling the phase of theinitial pulse (e.g. between ø+90° and ø−90° in a pair of measurements),will likewise cycle the phase of the echo signal by 180 degrees betweenmeasurements. The secondary transverse magnetization signal and othersignals which do not depend on the phase of the initial transmit pulsewill be unaffected by the phase cycling and will maintain constant phasefor both measurements. Thus using a method such as illustrated in FIG.5, in which the phase of the excitation pulse is cycled between ø+90 andø−90 and the refocusing pulse has a fixed phase of ø, as shown in thetwo top graphs, provides two recorded signals pairs, shown at bottomleft and bottom middle, which can be subtracted to isolate the echosignal from the secondary transverse magnetization signal, as shown inthe bottom right graph.

In some embodiments, a spin echo pulse sequence may comprise thetransmission of a first pulse, followed by a time delay that is not longin comparison to the transverse relaxation time (T2) for the NMR processof interest, followed by a second transmitted pulse which has a pulsemoment approximately twice that of the first pulse, followed bydetection of a delayed and desired spin echo signal. In some embodimentsof the spin echo pulse sequence with phase cycling, the phase of thesecond pulse in the two-pulse sequence may be shifted by 90 degrees or180 degrees with respect to the phase of the first pulse.

In some embodiments, a two-pulse spin echo sequence may be applied topreserve the desired spin echo signals after each refocusing pulse, andcancel undesired signals that exhibit a constant phase. Such undesiredsignals may include instrumentation or electronics-related transientsignals that exhibit a signal or response that is independent of thetransmitted pulse, and/or FID NMR signals initiated by the excitation orrefocusing pulses. For example, a spin echo pulse sequence may consistof an excitation transmit pulse, followed by a finite time delay,followed by a refocusing pulse wherein NMR signal acquisition may occurafter the excitation and/or refocusing pulses. The transmitted energy ofthe refocusing pulse may be arbitrary relative to that of the excitationpulse, or may be twice that of the excitation pulse. The phase shiftbetween the excitation and refocusing pulses may be arbitrary. Thistwo-pulse sequence may be applied twice. In the second application ofthe two-pulse sequence, the phase of first transmitted pulse may beshifted by substantially 180 degrees with respect to the phase of thefirst transmitted pulse in the first application of the sequence. Eachtwo-pulse sequence may be followed by detection and recording of theresultant NMR signal.

SNMR processing modules may be configured to process the data sets fromthe two applied spin echo sequences, e.g. by subtracting the data sets,yielding one linearly combined data set. Since the phase of the NMR spinecho signal tracks the phase of the excitation pulse, and may be shiftedby substantially 180 degrees in the two individual data sets, in thelinearly combined data set the desired NMR spin echo signals addconstructively and are hence preserved. Any undesired signals that arethe same for the two applied signals including electronic switchingtransients that are independent of the transmitted pulse phase,transient signals from eddy currents associated with rapid terminationof the refocusing pulse, and/or any FID signals initiated by therefocusing pulse may be cancelled to zero in the linearly combined dataset.

In some embodiments, SNMR processing modules may be configured toprocess a linearly combined dataset, in which the echo signals arepreserved, to estimate the localized NMR relaxation parameters T2*and/or T2.

In some embodiments, SNMR processing modules may be configured toprocess a linearly combined data set by adding, rather than subtracting,the datasets from the two applied spin echo sequences. In such alinearly combined dataset, the spin echo signals are cancelled and theFID signals initiated by the refocusing pulses are preserved because theFID signals initiated by the refocusing pulse have consistent phase inthe two individual data sets and so add constructively and are hencepreserved. SNMR processing modules may be configured to use a linearlycombined dataset, in which the FID signals initiated by the refocusingpulse are preserved, to estimate the localized NMR relaxation parametersT1 and/or T2*.

In some embodiments, spin echo acquisition may comprise 4-step spin echophase cycling to isolate the spin echoes signals as well as FID signalsfrom the first pulse and refocusing pulse, or “4-step spin echo”. In a4-step spin echo acquisition the two-pulse spin echo sequence may beapplied four times with similar cycling of transmitted pulse phases asdescribed previously for the 4-step pseudo saturation-recovery. In thefirst application of the two-pulse sequence, the transmitted phases ofthe first and second pulses may be arbitrary. In the second applicationof the two-pulse sequence, the phase of first transmitted pulse may beshifted by substantially 180 degrees with respect to the phase of thefirst transmitted pulse in the first application of the sequence. In thethird application of the two-pulse sequence, the phase of secondtransmitted pulse may be shifted by substantially 180 degrees withrespect to the phase of the second transmitted pulse in the firstapplication of the sequence. In the fourth application of the two-pulsesequence, the phase of first transmitted pulse may be shifted bysubstantially 180 degrees with respect to the phase of the firsttransmitted pulse in the first application of the sequence, and phase ofsecond transmitted pulse may be shifted by substantially 180 degreeswith respect to the phase of the second transmitted pulse in the firstapplication of the sequence. Four sets of data may be received andrecorded, and the relative phase shifts of the second FID signalφ_(FID2) and second FID signal φ_(ECHO) for each data set for each dataset, denoted as (φ_(FID1), φ_(FID2), φ_(ECHO)) may be as follows: A1=(0,0, 0), A2=(pi, 0, pi), A3=(0, pi, 0), A4=(pi, pi, pi). The order inwhich the four sequences are acquired is arbitrary. On sequence A1, thephase of the first transmit pulse relative to the second transmit pulseis also arbitrary.

SNMR processing modules may be configured to process the data sets from4-step pseudo-saturation-recovery by linearly combining the four sets ofdata in different ways to isolate different desired and/or undesiredsignals. For example, in a first linear combination C1, the second dataset A2=(pi, 0, pi) may be subtracted from the first data set A1=(0, 0,0) to yield a single combined data set in which the spin echo signalsand first FID signals are preserved with zero phase shift, and anyundesired signals with constant envelope and phase including the FIDfrom the second pulse may be canceled or reduced. This result is denotedC1=(0, X, 0), wherein the “X” indicated that the second FID signal hasbeen cancelled, and the phase of the combined first FID signal and spinecho signal are zero.

In a second combination C2, the third data set A3=(0, pi, 0) may besubtracted from the fourth data set A4=(pi, pi, pi) to yield anothersingle combined data set, C2=(pi, X, pi), in which the spin echo signalsand first FID signals are preserved with zero phase shift and anyundesired signals with constant envelope and phase including the FIDfrom the second pulse may be canceled or reduced.

In a third combination C3, the third data set A3=(0, pi, 0) may besubtracted from the first data set A1=(0, 0, 0) to yield a singlecombined data set in which the FID signal initiated by the secondtransmit pulse is preserved with a phase of 0 degrees, and any undesiredsignals with constant envelope and phase following the second pulse,including instrumentation artifacts, spin echo signals, and/or anyremnant FID signal initiated by the first pulse, may be canceled orreduced. This result is denoted C3=(X, 0, X), wherein the “X” indicatedthat the first FID signal as well as any echo signals have beencancelled, and the phase of the combined second FID signal is zero.

In a fourth combination C4, the second data set A2=(pi, 0, pi) may besubtracted from the fourth data set A4=(pi, pi, pi) to yield a singlecombined data set, C4=(X, pi, X), in which the FID signal initiated bythe second transmit pulse is preserved with a phase shift of pi, and anyundesired signals with constant envelope and phase following the secondpulse, including instrumentation artifacts, spin echo signals, and/orany remnant FID signal initiated by the first pulse, may be canceled orreduced.

In a fifth combination C5, the fourth data set A4=(pi, pi, pi) may besubtracted from the first data set A1=(0, 0, 0) to yield a singlecombined data set in which the FID signal initiated by the firsttransmit pulse, the FID signal initiated by the second pulse and thespin echo signal are all preserved with a phase shift of 0, and anyundesired signals with constant envelope and phase following the secondpulse, including instrumentation artifacts may be canceled or reduced.This result is denoted C5=(0, 0, 0).

In a sixth combination C6, the third data set A3=(0, pi, 0) may besubtracted from the second data set A2=(pi, 0, pi) to yield a singlecombined data set in which the FID signal initiated by the firsttransmit pulse and the spin echo signal are both preserved with a phaseshift of pi, the FID signal initiated by the second pulse is preservedwith a phase shift of 0, and any undesired signals with constantenvelope and phase, including instrumentation, may be canceled orreduced. This result is denoted C6=(pi, 0, pi).

The first six linear combinations described above yield six combinationsof the desired FID signals while undesired instrumentation artifacts andother undesired signals with constant envelope and phase are canceled orreduced: C1=(0, X, 0), C2=(pi, X, pi), C3=(X, 0, X), C4=(X, pi, X),C5=(0, 0, 0), C6=(pi, 0, pi). These six linear combinations {C1, C2, C3,C4, C5, C6} may also be linearly combined in different ways to furtherenhance one or both of the desired FID signals. For example, thecombinations C1 and C2 may be further combined via the equationD1=(C1−C2)/2, such that the first FID signal and spin echo signal arepreserved with zero phase shift and white Gaussian measurement noise isreduced by a factor of sqrt(2). Similarly the combinations C3 and C4 maybe further combined via the equation D2=(C3−C4)/2, such that the secondFID signal is preserved with zero phase shift, any remnant FID signalinitiated by the first pulse or spin echo signals are cancelled, andwhite Gaussian measurement noise is reduced by a factor of sqrt(2).

Additional combinations of the acquired data (A1, A2, A3, A4) and/orlinear combinations of the acquired data (C1, C2, etc. . . . ) may bedeveloped and applied to enhance other desired NMR signal processes ornon-NMR signals of interest, and suppress specific undesired NMR andnon-NMR signals.

The aforementioned embodiments of the phase-cycled pseudo saturationrecovery and the phase-cycled spin echo use similar phase variations forthe transmitted pulses. The primary deference between the twoacquisition methods is the amplitude of transmitted energy of the secondpulse relative to the first pulse. In some embodiments of the pseudosaturation recovery sequence, the transmitted energy of the second pulsemay be approximately equal to that of the first pulse. In someembodiments of the spin echo sequence, the transmitted energy of thesecond pulse may be approximately twice that of the first pulse. Inother embodiments, the same general 4-step phase cycling approach may beused for a general acquisition scheme in which the transmitted energy ofthe second pulse may be arbitrary relative to that of the first pulse.Data produced from such embodiments, in which the transmitted energy ofthe two pulses is arbitrary, can also be used to record and isolate FIDand spin echo signals and to localize and estimate the abundance ofsubsurface fluids and also the T2* and/or T1 relaxation processes of thedetected subsurface fluids.

In some pulse sequence configurations, at least two transmittedelectrical current pulse sequences may each comprise aCarr-Purcell-Meiboom-Gill (CPMG) pulse sequence. TheCarr-Purcell-Meiboom-Gill (CPMG) pulse sequences may comprise anoscillating electrical current excitation pulse, followed by a series ofone or more time delays and oscillating electrical current refocusingpulses, each refocusing pulse having a pulse moment approximately twiceas large as the excitation pulse, and each refocusing pulse having aphase substantially equal to the phase of the other refocusing pulseswithin a same pulse sequence. The phase of either an oscillatingelectrical current excitation pulse, or the refocusing pulses applied inat least one of the CPMG pulse sequences may differ by substantially 180degrees from the phase of an oscillating electrical current excitationpulse, or the refocusing pulses, respectively, applied in at leastanother of the CPMG pulse sequences. This approach may be referred toherein as phase cycled CPMG acquisition.

In some embodiments, a phase cycled CPMG pulse sequence may comprise theapplication of a first pulse with finite energy, followed by apredetermined time delay, followed by a set of refocusing pulses, eachof said refocusing pulses having energy approximately twice that of thefirst pulse, and wherein each of the refocusing pulses are separated bytime delays equal to twice the initial delay period. The CPMG pulsesequence causes NMR spin echo signals to refocus in the time intervalsapproximately midway between each of the refocusing pulses. The CPMGsequence is useful for estimating the abundance of subsurface fluid andits distribution in different T2 relaxation times.

In some embodiments, a phase cycled CPMG pulse sequence may comprise afirst transmitted pulse (excitation pulse) is applied to generate atransverse magnetization, followed by a plurality of time delays andrefocusing pulses, where each refocusing pulse has a substantiallyidentical transmit phase. The transmitted energy of the individualrefocusing pulses may be arbitrary, or may be twice as large as that ofthe first transmitted excitation pulse. In this embodiment, two separateCPMG acquisitions may be performed, wherein the phase of the excitationpulse in the second acquisition is shifted by substantially 180 degreesrelative to the phase of the excitation pulse in the first acquisition.Each CPMG pulse sequence may be followed by detection and recording ofthe resultant desired FID signal.

SNMR processing modules may be configured to process the data sets fromthe CPMG pulse sequences. Recorded NMR signal data from the secondacquisition may be coherently subtracted from the NMR signal data fromthe first acquisition, to produce a final NMR signal. In this final NMRsignal, the desired spin echo NMR signals are preserved, and anyundesired signals that are coherent with the timing of the acquisitionsbut independent of the transmitted pulse phases are cancelled. Inaddition, in the final NMR signal any additional undesired NMR signalsthat are in phase with the refocusing pulses, including spurious FIDsignals generated by the refocusing pulses, are cancelled.

FIG. 3 is a flow diagram illustrating example SNMR pulse sequence phasecycling methods. The various elements illustrated in FIG. 3 illustrateboth operations that may be performed in a method, and modules as may beincluded in a computing device 110. These include an “Apply PulseSequence” block 301, a “Record NMR Data” block 311, a “Change Phase ofOne or More Pulses, and Re-Apply Pulse Sequence” block 302, a “RecordNMR Data” block 312, a “Change Phase of One or More Pulses, and Re-ApplyPulse Sequence” block 30(N), a “Record NMR Data” block 31(N). In FIG. 3,operations 302 and 312 may be repeated N times, where N can be anynumber from zero to any desired number of repetitions. Each of the“Record NMR Data” blocks 311, 312, 31(N) produce stored data 321, 322,and 32(N), respectively. The stored data is processed for example bymultipliers 342, 343, and 34(N), which multiply by the real or complexscalars 331, 332, and 33(N). The output of the multipliers 342, 343, and34(N) may be summed by summing operator 380, which produces stored dataoutput 390.

In some embodiments according to FIG. 3, a phase cycled SNMR dataacquisition and recombination set may consist of two phase cycles, e.g.,two pulse sequences in which the phase of one or more pulses in thesecond pulse sequence is different from a phase of a corresponding pulsein the first pulse sequence. A first SNMR pulse sequence is applied 301,resulting in the recording 311 and storage of a first data set 321. Thephase of one or more of the pulses in the pulse sequence 301 is changedand the pulse sequence is reapplied 302 resulting in the recording 312and storage of a second data set 322. The relative phases of the desiredNMR signal and undesired signal(s) are different in the two data sets321 and 322, due to the change of the phase in the one or more transmitpulses between the first pulse sequence 301 and second pulse sequence302. The two data sets 321 and 322, may be linearly combined into asingle data set via multiplication 342 and 343 by the real or complexscalars 331 and 332, and subsequent summation 380. In the combined andstored data set 390, a desired NMR signal is preserved and one or moreundesired signals(s) are canceled or reduced.

In some embodiments according to FIG. 3, a phase cycled SNMR dataacquisition and recombination set may comprise more than two phasecycles, e.g., more than two pulse sequences in which the phase of one ormore pulses in at least one of the subsequent pulse sequences isdifferent from a phase of a corresponding pulse in a previous pulsesequence. For example, a SNMR pulse sequence may be applied N timesusing N unique combinations of phases on the individual pulses in thepulse sequence (301, 302, 30(N)), resulting in the recording (311, 312,31(N)) and storage of N data sets (321, 322, 32(N)). The relative phasesof the desired NMR signal and undesired signal(s) are different amongthe plurality of stored data sets (321, 322, 32(N)) due to the uniquecombinations of transmit pulse phases in the plurality of applicationsof the pulse sequence (301, 302, 30(N)). The N data sets (321, 322,32(N)) may be linearly combined into a single data set viamultiplication (342, 343, 34(N)) by the real or complex scalars (331,332, 33(N)) and subsequent summation 380. In the combined and storeddata set 390, a desired NMR signal is preserved and one or moreundesired signals(s) are canceled or reduced.

In some embodiments, the operations illustrated in FIG. 3 may beseparated into acquisition and processing, e.g., by first carrying outthe acquisition of NMR data, then performing the processing either at alater time or with a different device. Also, the operations illustratedin FIG. 3 may be combined with any number of operations involved inperforming SNMR measurements using a system such as illustrated inFIG. 1. For example, SNMR detection operations may generally comprisedeploying one or more induction coils on or above the surface of theEarth for use as magnetic field transmitting sources and/or magneticfield detection devices, deploying one or more induction coils on orabove the surface of the Earth for use as magnetic field detectiondevices, transmitting a sequence of one or more current pulses throughone or more of the transmitting coils at or near the Larmor frequency ofthe Earth's local magnetic field, to cause a coherent precession of NMRspins in fluid(s) in the subsurface, and/or using the one or moredetection coils to detect the alternating magnetic field caused by theprecessing NMR spins in the subsurface fluid(s).

A single induction coil may be used for both transmitting and detection,or separate coils may be used for transmitting and detection functions.The SNMR detection methods disclosed herein may be employ multipletransmit and detection coils, for example as disclosed in U.S. Pat. No.7,466,128. Various methods have been developed for localizing NMRsignals acquired via SNMR detection techniques, and these localizationmethods have been applied to localize NMR signals in one, two or threedimensions. Various methods have also been developed and applied toestimate aquifer and reservoir properties based on NMR data obtainedusing the SNMR detection technique, and such methods may also becombined with the operations disclosed herein.

In some embodiments, the SNMR methods disclosed herein may be used todetect fluids beneath the surface of the Earth, including groundwaterand hydrocarbon fluids. SNMR techniques may also applicable to detectionof fluids beneath and within man-made structures, including earthen orconcrete dams, levees, mine tailing piles, piles of raw or processedmaterials, and landfills. SNMR methods are also potentially useful fordetecting fluids beneath the surfaces of extraterrestrial bodies,including nearby planets such as Mars. In the extraterrestrialapplication, the method would rely upon a local static magnetic fieldproduced by the extraterrestrial body itself, rather than the Earth'smagnetic field.

In some embodiments, SNMR methods may generally comprise thetransmission of a specific sequence of pulses, to activate NMR signalprocesses in the Earth's magnetic field, and the simultaneous detectionof desired NMR signals due to fluids in the subsurface. The SNMRdetection method may thus produce data that is subsequently useful foranalysis of distribution of fluid content in the subsurface.

In some embodiments, the SNMR methods disclosed herein may separate“desired” from “undesired” coherent signals. The desired signals arethose particular coherent NMR signals emitted by subsurface liquids thatcan be analyzed to determine the NMR properties of the subsurface. Theundesired signals are any coherent signals that interfere with therecording of the desired signal or the determination of subsurface NMRproperties, and may include signals from non-NMR sources as wellinterfering signals from NMR sources. In particular, non-NMR sources ofundesired signals may include transient responses of the SNMR detectionapparatus that do not necessarily follow the amplitude and phase of thetransmitted pulse(s). Sources of these types of non-NMR signals mayinclude switching devices, transient responses of electrical circuits,or any other source of signal that is timed-locked with the SNMRdetection sequence but not necessarily dependent on the transmittedpulse or pulses. Non-NMR sources of undesired signals may also includetransient responses of the subsurface associated with eddy currentsinduced by the rapid shut-off of the transmitted pulse(s).

NMR-related sources of undesired signals may include remnant FIDsignals, and/or undesired stimulated or refocused NMR signals. Forexample in a spin-echo sequence, the application of the second pulse cangenerate a FID signal which can interfere with the detection andinterpretation of the desired spin echo signal or signals. Also, in apseudo-inversion-recovery sequence, the FID signal from the first pulsecan overlap in time with the desired FID signal from the second pulse,thus interfering with detection and interpretation of the second pulseFID signal.

The various desired and undesired signal components may have similarspectral properties and may overlap in the time domain as well, so it isgenerally difficult to separate the desired signals from those which areundesired. In particular, the presence of undesired transient signalsthat are concentrated at the beginning of the recorded data can inhibitthe ability to detect and interpret the desired early time NMR signals.The disclosed methods may be employed to robustly suppress the undesiredsignals and preserve the desired NMR signals that can be used todetermine the NMR properties of the subsurface.

In some embodiments, the SNMR methods disclosed herein may comprisemethods configured to acquire SNMR signals in a set of two or morerepeated acquisition sequences, also referred to herein as pulsesequences, involving a phase shift on one or more of the transmittedpulses in one or more of the acquisition sequences. Disclosed methodsmay comprise, alternatively or in addition to the SNMR acquisition, SNMRprocessing including linearly combining the signals recorded during thetwo or more acquisition sequences so as to suppress an undesired signalor signals and to preserve a desired signal or signals. We define phasecycling as the process of performing two or more repeated SNMRmeasurements, wherein the phase of one or more of the pulses in thesequence is varied between the two or more repeated measurements.Embodiments of this disclosure can also be used to cancel or reduce oneor more NMR signals for the purpose of detecting and isolating one ormore non-NMR signals.

In some embodiments, a single acquisition sequence may comprise (1)transmitting one or more pulses in a sequence of short succession on oneor more transmit coils (2) recording the voltage signal on one or morereceive coils after one of more transmit pulses and (3) waiting a periodof time after the final pulse in the sequence to allow the subsurfaceNMR state to return to equilibrium, which may be equal to or greaterthan the T1 relaxation process for the subsurface liquids of interest. Acycled set of sequences may be comprised of two or more similaracquisition sequences between which a controlled phase shift is appliedto one or more transmit pulses.

In some embodiments, a SNMR detection sequence may be designed orselected so as to detect a desired NMR signal. One or more undesiredsignals or processes may be identified. A phase alternating set ofacquisitions may be designed so as to preserve the desired NMR signalwhile canceling or reducing the undesired signal(s). The selected pulsesequence may be performed two or more times wherein the phase of atleast one pulse in the selected sequence is varied between at least twoof the performed pulse sequences, resulting in two or more data sets.The two or more data sets may be linearly combined into a single dataset that preserves the desired NMR signal and cancels or reduces the oneor more undesired signals.

In some embodiments, the process of collecting data in theaforementioned phase cycled manner, and recombining the phase cycleddata, may employ any linear combination of the recorded data that causesa desired NMR signal to be preserved and one or more undesired signalsto be canceled or reduced. In particular, the linear combination mayinvolve subtracting one or more of the sampled data sets from other datasets or combinations thereof, phase shifting individual data sets viamultiplication by complex scalar values and then performing addition orsubtraction among various data sets, and linearly combining differentsamples in time.

There is little distinction left between hardware and softwareimplementations of aspects of systems; the use of hardware or softwareis generally (but not always) a design choice representing cost vs.efficiency tradeoffs. There are various vehicles by which processesand/or systems and/or other technologies described herein can beeffected (e.g., hardware, software, and/or firmware), and that thepreferred vehicle may vary with the context in which the processesand/or systems and/or other technologies are deployed. For example, ifan implementer determines that speed and accuracy are paramount, theimplementer may opt for a mainly hardware and/or firmware vehicle; ifflexibility is paramount, the implementer may opt for a mainly softwareimplementation; or, yet again alternatively, the implementer may opt forsome combination of hardware, software, and/or firmware.

The foregoing detailed description has set forth various embodiments ofthe devices and/or processes via the use of block diagrams, flowcharts,and/or examples. Insofar as such block diagrams, flowcharts, and/orexamples contain one or more functions and/or operations, it will beunderstood by those within the art that each function and/or operationwithin such block diagrams, flowcharts, or examples can be implemented,individually and/or collectively, by a wide range of hardware, software,firmware, or virtually any combination thereof. In one embodiment,several portions of the subject matter described herein may beimplemented via Application Specific Integrated Circuits (ASICs), FieldProgrammable Gate Arrays (FPGAs), digital signal processors (DSPs), orother integrated formats. However, those skilled in the art willrecognize that some aspects of the embodiments disclosed herein, inwhole or in part, can be equivalently implemented in integratedcircuits, as one or more computer programs running on one or morecomputers (e.g., as one or more programs running on one or more computersystems), as one or more programs running on one or more processors(e.g., as one or more programs running on one or more microprocessors),as firmware, or as virtually any combination thereof, and that designingthe circuitry and/or writing the code for the software and or firmwarewould be within the skill of one skilled in the art in light of thisdisclosure. In addition, those skilled in the art will appreciate thatthe mechanisms of the subject matter described herein are capable ofbeing distributed as a program product in a variety of forms, and thatan illustrative embodiment of the subject matter described hereinapplies regardless of the particular type of signal bearing medium usedto actually carry out the distribution. Examples of a signal bearingmedium include, but are not limited to, the following: a recordable typemedium such as a floppy disk, a hard disk drive, a Compact Disc (CD), aDigital Video Disk (DVD), a digital tape, a computer memory, etc.; and atransmission type medium such as a digital and/or an analogcommunication medium (e.g., a fiber optic cable, a waveguide, a wiredcommunications link, a wireless communication link, etc.).

Those skilled in the art will recognize that it is common within the artto describe devices and/or processes in the fashion set forth herein,and thereafter use engineering practices to integrate such describeddevices and/or processes into data processing systems. That is, at leasta portion of the devices and/or processes described herein can beintegrated into a data processing system via a reasonable amount ofexperimentation. Those having skill in the art will recognize that atypical data processing system generally includes one or more of asystem unit housing, a video display device, a memory such as volatileand non-volatile memory, processors such as microprocessors and digitalsignal processors, computational entities such as operating systems,drivers, graphical user interfaces, and applications programs, one ormore interaction devices, such as a touch pad or screen, and/or controlsystems including feedback loops and control motors (e.g., feedback forsensing position and/or velocity; control motors for moving and/oradjusting components and/or quantities). A typical data processingsystem may be implemented utilizing any suitable commercially availablecomponents, such as those typically found in datacomputing/communication and/or network computing/communication systems.The herein described subject matter sometimes illustrates differentcomponents contained within, or connected with, different othercomponents. It is to be understood that such depicted architectures aremerely exemplary, and that in fact many other architectures can beimplemented which achieve the same functionality. In a conceptual sense,any arrangement of components to achieve the same functionality iseffectively “associated” such that the desired functionality isachieved. Hence, any two components herein combined to achieve aparticular functionality can be seen as “associated with” each othersuch that the desired functionality is achieved, irrespective ofarchitectures or intermediate components. Likewise, any two componentsso associated can also be viewed as being “operably connected”, or“operably coupled”, to each other to achieve the desired functionality,and any two components capable of being so associated can also be viewedas being “operably couplable”, to each other to achieve the desiredfunctionality. Specific examples of operably couplable include but arenot limited to physically mateable and/or physically interactingcomponents and/or wirelessly interactable and/or wirelessly interactingcomponents and/or logically interacting and/or logically interactablecomponents.

With respect to the use of substantially any plural and/or singularterms herein, those having skill in the art can translate from theplural to the singular and/or from the singular to the plural as isappropriate to the context and/or application. The varioussingular/plural permutations may be expressly set forth herein for sakeof clarity.

It will be understood by those within the art that, in general, termsused herein, and especially in the appended claims (e.g., bodies of theappended claims) are generally intended as “open” terms (e.g., the term“including” should be interpreted as “including but not limited to,” theterm “having” should be interpreted as “having at least,” the term“includes” should be interpreted as “includes but is not limited to,”etc.). It will be further understood by those within the art that if aspecific number of an introduced claim recitation is intended, such anintent will be explicitly recited in the claim, and in the absence ofsuch recitation no such intent is present. For example, as an aid tounderstanding, the following appended claims may contain usage of theintroductory phrases “at least one” and “one or more” to introduce claimrecitations. However, the use of such phrases should not be construed toimply that the introduction of a claim recitation by the indefinitearticles “a” or “an” limits any particular claim containing suchintroduced claim recitation to inventions containing only one suchrecitation, even when the same claim includes the introductory phrases“one or more” or “at least one” and indefinite articles such as “a” or“an” (e.g., “a” and/or “an” should typically be interpreted to mean “atleast one” or “one or more”); the same holds true for the use ofdefinite articles used to introduce claim recitations. In addition, evenif a specific number of an introduced claim recitation is explicitlyrecited, those skilled in the art will recognize that such recitationshould typically be interpreted to mean at least the recited number(e.g., the bare recitation of “two recitations,” without othermodifiers, typically means at least two recitations, or two or morerecitations). Furthermore, in those instances where a conventionanalogous to “at least one of A, B, and C, etc.” is used, in generalsuch a construction is intended in the sense one having skill in the artwould understand the convention (e.g., “a system having at least one ofA, B, and C” would include but not be limited to systems that have Aalone, B alone, C alone, A and B together, A and C together, B and Ctogether, and/or A, B, and C together, etc.). In those instances where aconvention analogous to “at least one of A, B, or C, etc.” is used, ingeneral such a construction is intended in the sense one having skill inthe art would understand the convention (e.g., “a system having at leastone of A, B, or C” would include but not be limited to systems that haveA alone, B alone, C alone, A and B together, A and C together, B and Ctogether, and/or A, B, and C together, etc.). It will be furtherunderstood by those within the art that virtually any disjunctive wordand/or phrase presenting two or more alternative terms, whether in thedescription, claims, or drawings, should be understood to contemplatethe possibilities of including one of the terms, either of the terms, orboth terms. For example, the phrase “A or B” will be understood toinclude the possibilities of “A” or “B” or “A and B.”

While various embodiments have been disclosed herein, other aspects andembodiments will be apparent to those skilled in art.

1. A Surface Nuclear Magnetic Resonance (SNMR) method, comprising:arranging one or more induction coils on the surface of the Earth;transmitting two or more electrical current pulse sequences on theinduction coils, wherein each of the transmitted pulse sequences areassociated with a single NMR measurement, wherein each of thetransmitted pulse sequences comprises one or more oscillating electricalcurrent pulses, and wherein each of the oscillating electrical currentpulses has a phase of oscillation relative to the other oscillatingelectrical current pulses in the two or more transmitted pulsesequences; applying a phase shift to at least one of the transmittedoscillating electrical current pulses in at least one of the transmittedpulse sequences, wherein the applied phase shift is relative to a phaseof at least one of the transmitted oscillating electrical current pulsesin another of the transmitted pulse sequences; detecting signals on theinduction coils after and/or during each of the electrical current pulsesequences; and linearly combining detected signal data corresponding toseparate electrical current pulse sequences to produce combined signaldata in which one or more detected signal components are preserved andone or more different detected signal components are reduced orcancelled.
 2. The SNMR method of claim 1, wherein the preserved signalcomponents comprise NMR signal data, and wherein the reduced orcancelled signal components comprise one or more of undesired NMR signaldata and non-NMR signal data.
 3. The SNMR method of claim 1, wherein thepreserved signal components comprise one or more of undesired NMR signaldata and non-NMR signal data, and wherein the reduced or cancelledsignal components comprise NMR signal data.
 4. The SNMR method of claim1: wherein at least two of the transmitted electrical current pulsesequences each consist of a single oscillating electrical current pulse;and wherein the phase of a single oscillating electrical current pulseapplied in at least one of the single pulse sequences differs bysubstantially 180 degrees from the phase of a single oscillatingelectrical current pulse applied in at least another of the single pulsesequences.
 5. The SNMR method of claim 1: wherein at least two of thetransmitted electrical current pulse sequences each consist of twooscillating electrical current pulses separated by a time delay.
 6. TheSNMR method of claim 5: wherein the phase of the first oscillatingelectrical current pulse applied in at least one of the two-pulse pulsesequences differs by substantially 180 degrees from the phase of thesecond electrical current pulse applied in at least another of thetwo-pulse pulse sequences; or wherein the phase of the secondoscillating electrical current pulse applied in at least one of thetwo-pulse pulse sequences differs by substantially 180 degrees from thephase of the second electrical current pulse applied in at least anotherof the two-pulse pulse sequences; or wherein the phase of the first andsecond oscillating electrical current pulses applied in at least one ofthe two-pulse pulse sequences each differ by substantially 180 degreesfrom the phase of the first and second electrical current pulses,respectively, applied in at least another of the two-pulse pulsesequences.
 7. The SNMR method of claim 5: wherein at least four of thetransmitted electrical current pulse sequences each consist of twooscillating electrical current pulses separated by a time delay; whereineach of the four two-pulse pulse sequences comprises a first oscillatingelectrical current pulse with a first phase, and a second oscillatingelectrical current pulse with a second phase; wherein in one of the fourtwo-pulse pulse sequences, the first phase is substantially at a definedreference first phase and second phase is substantially at a definedreference second phase; wherein in another of the four two-pulse pulsesequences, the first phase is substantially at the defined referencefirst phase, and second phase is shifted substantially 180 degrees fromthe defined reference second phase; wherein in another of the fourtwo-pulse pulse sequences, the first phase is shifted substantially 180degrees from the defined reference first phase, and second phase issubstantially at the defined reference second phase; and wherein inanother of the four two-pulse pulse sequences, the first phase isshifted substantially 180 degrees from the defined reference firstphase, and second phase is shifted substantially 180 degrees from thedefined reference second phase.
 8. The SNMR method of claim 5, whereinat least two of the oscillating electrical current pulses in each of thepulse sequences is separated by a time delay that is shorter than thetime for an underground liquid to achieve substantially completelongitudinal and/or transverse relaxation.
 9. The SNMR method of claim5, wherein the oscillating electrical current pulses in the two-pulsepulse sequences have substantially equal moment; or wherein theamplitude of the first oscillating electrical current pulse in thetwo-pulse sequence is substantially greater than the pulse moment of thesecond oscillating electrical current pulse; or wherein the amplitude ofthe first oscillating electrical current pulse in the two-pulse sequenceis substantially less than the pulse moment of the second oscillatingelectrical current pulse.
 10. The SNMR method of claim 5, wherein atleast two of the transmitted electrical current pulse sequences eachcomprise a spin-echo pulse sequence, the spin-echo pulse sequencescomprising an oscillating electrical current excitation pulse, followedby a time delay, followed by an oscillating electrical currentrefocusing pulse; and wherein the phase of an oscillating electricalcurrent excitation pulse applied in at least one of the spin-echo pulsesequences differs by substantially 180 degrees from the phase of anoscillating electrical current excitation pulse applied in at leastanother of the spin-echo pulse sequences.
 11. The SNMR method of claim1: wherein at least two of the transmitted electrical current pulsesequences each consist of N oscillating electrical current pulses, wereN is greater than one, and wherein each of the pulse sequences areseparated by a time delay; and wherein the phase of any i^(th)oscillating electrical current pulse applied in at least one of theN-pulse pulse sequences differs by substantially 180 degrees from thephase of an i^(th) oscillating electrical current pulse applied in atleast another of the N-pulse pulse sequences.
 12. The SNMR method ofclaim 1: wherein at least two of the transmitted electrical currentpulse sequences each comprise a Carr-Purcell-Meiboom-Gill (CPMG) pulsesequence, the CPMG pulse sequences comprising an oscillating electricalcurrent excitation pulse, followed by a series of one or more timedelays and oscillating electrical current refocusing pulses, eachrefocusing pulse having a pulse moment approximately twice as large asthe excitation pulse, and each refocusing pulse having a phasesubstantially equal to the phase of the other refocusing pulses within asame pulse sequence; and wherein the phase of either an oscillatingelectrical current excitation pulse, or the refocusing pulses applied inat least one of the CPMG pulse sequences differs by substantially 180degrees from the phase of an oscillating electrical current excitationpulse, or the refocusing pulses, respectively, applied in at leastanother of the CPMG pulse sequences.
 13. The SNMR method of claim 1,wherein the oscillating electrical current excitation pulses generate atransverse nuclear magnetization in an underground liquid.
 14. The SNMRmethod of claim 13, further comprising applying linear spatial inversionprocessing, non-linear spatial inversion processing, orcorrelation-based spatial processing, to localize detected NMR signalsfrom underground liquids.
 15. The SNMR method of claim 14, furthercomprising using the localized detected NMR signals to estimateunderground fluid content and its spatial distribution.
 16. The SNMRmethod of claim 15, further comprising using estimated underground fluidcontent and its spatial distribution to estimate properties offluid-bearing underground formations, including groundwater aquifers andhydrocarbon reservoirs.
 17. The SNMR method of claim 16, furthercomprising using the localized detected NMR signals to estimatelocalized NMR relaxation parameters comprising one or more oflongitudinal relaxation T1, FID relaxation rate T2*, and/or spin-spinrelaxation T2.
 18. A Surface Nuclear Magnetic Resonance (SNMR) systemconfigured to produce NMR in underground liquids, comprising: a SNMRphase cycling computer comprising a processor and memory; the SNMR phasecycling computer comprising one or more SNMR phase cycling acquisitionmodules configured to control transmitting of two or more electricalcurrent pulse sequences on induction coils arrangeable on or above thesurface of the Earth, wherein each of the transmitted pulse sequencesare associated with a single NMR measurement, wherein each of thetransmitted pulse sequences comprises one or more oscillating electricalcurrent pulses, and wherein each of the oscillating electrical currentpulses has a phase of oscillation relative to the other oscillatingelectrical current pulses in the two or more electrical current pulsesequences; and one or more SNMR phase cycling acquisition modulesconfigured to apply a phase shift to at least one of the transmittedoscillating electrical current pulses in at least one of the transmittedpulse sequences, wherein the applied phase shift is relative to a phaseof at least one of the transmitted oscillating electrical current pulsesin another of the transmitted pulse sequences.
 19. The SNMR system ofclaim 18, wherein the controller is configured to control transmittingof two or more electrical current pulse sequences via a power amplifiercoupled with the induction coils, and wherein the controller isconfigured to apply the phase shift by switching a power amplifier inputbetween oscillating waveform generator devices.
 20. The SNMR system ofclaim 18, wherein the oscillating waveform generator devices compriseone or more of a function generator and a computer, wherein the computeris equipped with waveform generating software and a digital to analogsignal converter.